Electrographic printers, photocopy machines, facsimile machines and the like are examples of an image forming apparatus that transfers a black-and-white or color toner image formed on a toner image forming part onto a medium, such as a sheet and the like. Such an image forming apparatus includes a fixing device. Conventionally, this type of fixing device includes a movable endless fixing belt, a sheet heating body that heats the fixing belt, a fixing roller that stretches the fixing belt with the sheet heating body, a pressure application roller provided to face the fixing roller via the fixing belt, and a nip part formed at a contact part of the pressure application roller and the fixing belt. The toner image is fixed on the medium by feeding the medium on which the toner image has been transferred to and passing through the nip part and by applying heat from the sheet heating body and pressure by the fixing roller and the pressure application roller at the time of passage (see Japanese Laid-Open Patent Application No. 2007-322888).
However, the above-described conventional technology does not consider a case in which an overtemperature protector is used for protecting the sheet heating body from overtemperature. One of objects of the present invention is to solve such a problem.